1. Field of the Invention
The present invention relates to a liquid ejection head for executing recording on a recording medium by ejecting liquid droplets such as, for example, ink droplets, and more particularly, to a liquid ejection head for executing inkjet recording.
2. Description of the Related Art
An inkjet recording system is one of so-called non-impact recording systems.
This inkjet recording system produces a negligible degree of small noise in recording and can execute recording at a high speed.
Further, the inkjet recording system can execute recording on various types of recording mediums, that is, it can fix ink on so-called plain paper without the need of special processing, and further can obtain a very fine image at a low cost. Recently, the inkjet recording system has become widely used rapidly as a recording means for a copier, facsimile, word processor, and the like due to these advantages, in addition to being used as a printer acting as a peripheral unit of a computer.
As an ink ejection method ordinarily used in the inkjet recording system, there are available a method of using an electrothermal transducer, for example, a heater, and a method of using a piezoelectric transducer, for example, a piezoelectric element, as an ejection energy generating element used to eject ink droplets. In any of the methods, the ejection of ink droplets can be controlled by an electric signal. A principle of the ink ejection method using the electrothermal transducer is that ink in the vicinity of the electrothermal transducer is instantly boiled by a voltage applied to the electrothermal transducer, and ink droplets are ejected at a high speed by the rapid growth of bubbles caused by the change of phase of ink in the boiling. In contrast, a principle of the ink ejection method using the piezoelectric transducer is that the piezoelectric transducer is displaced by a voltage applied thereto, and ink droplets are ejected by a pressure generated when the piezoelectric transducer is displaced.
The ink ejection method using the electrothermal transducer is advantageous in that a large space is not necessary to dispose an ejection energy generating element, the structure of the recording head is simple, and nozzles can be easily integrated. In contrast, a defect inherent to this ink ejection method resides in that the volume of a flying ink droplet fluctuates due to the heat generated by the electrothermal transducer and accumulated in the recording head, that the electrothermal transducer is adversely affected by cavitation generated when bubbles disappear, and that ink droplet ejection characteristics and image quality are adversely affected by air which is dissolved in the ink and remains in the recording head as remaining bubbles.
The inkjet recording methods and the recording heads disclosed in Japanese Patent Laid-Open Nos. 54-161935, 61-185455, 61-249768, and 4-10941 propose methods of solving these problems. That is, the inkjet recording methods disclosed in the publications described above are such that the bubbles generated by driving an electrothermal transducer in response to a recording signal are communicated with outside air. The employment of the image recording methods stabilizes the volume of a flying ink droplet, makes it to possible to eject a slight amount of an ink droplet at a high speed, and can improve the durability of a heater by eliminating cavitation generated when bubbles disappear, whereby a much finer image can be easily obtained. The publications described above exemplify an arrangement in which the shortest distance between an electrothermal transducer and an ejection port is greatly reduced compared to that of a conventional arrangement as an arrangement for communicating bubbles with outside air.
This type of a conventional recording head will be described below. The conventional recording head includes an element substrate on which electrothermal transducers for ejecting ink are disposed and a nozzle forming member laminated on the element substrate and constituting ink flow paths. The nozzle forming member includes a plurality of ejection ports for ejecting ink droplets, a plurality of nozzles through which ink flows, and a supply chamber for supplying ink to the respective nozzles. Each nozzle has a bubble forming chamber in which bubbles are generated by an electrothermal transducer and a supply path for supplying ink to the bubble forming chamber. The electrothermal transducers are disposed on the element substrate so as to be located in the bubble forming chambers. Further, a supply port is formed on the element substrate to supply ink to the supply chamber from the back surface side of the element substrate that is opposite to the main surface thereof adjacent to the nozzle forming member. Then, ejection ports are formed on the nozzle forming member at positions confronting the electrothermal transducers on the element substrate.
In the conventional recording head arranged as described above, the ink supplied from the supply port into the supply chamber is supplied along the respective nozzles and fill the bubble forming chambers. The ink having filled the bubble forming chambers is flown in a direction approximately perpendicular to the main surface of the element substrate by bubbles that are generated by a film boiling phenomenon caused by heat applied from the electrothermal transducers, and is ejected from the ejection ports as ink droplets.
Then, it is contemplated to further increase the recording speed of a recording apparatus provided with the recording head described above to output a recorded image of higher quality, an image of high quality, an image of high resolution, and the like. To increase the recording speed of a conventional recording apparatus, U.S. Pat. Nos. 4,882,595 and 6,158,843 disclose a trial for increasing the number of times of ejection of ink droplets flown from each nozzle of a recording head, that is, a trial for increasing an ejection frequency.
Further, in a conventional recording head, it is taken into consideration to improve an ejection efficiency such as an amount of ink droplets ejected from ejection ports, an ejection speed thereof, and the like and to improve a refill speed at which bubble forming chambers are filled with ink.
In general, when it is intended to improve the ejection efficiency, that is, the ejection characteristics of a recording head and the refill efficiency, it is important to infinitely increase an quantity of inertance from an electrothermal transducer to an ejection port as compared with a quantity of inertance from the electrothermal transducer to a supply port, as well as to reduce a resistance in a nozzle.
While the inertance and the resistance are varied by the length and cross sectional area of a nozzle, the ejection efficiency and the refill efficiency have been made full use such that they have moderate characteristics because the inertance and the resistance are in a relation of trade-off.
In contrast, high image quality and small droplets are more required from the recent trend of an inkjet system. Accordingly, it is desired to further improve the ejection efficiency and the refill efficiency from the view point of speed-up and energy-saving. However, the ejection efficiency and the refill efficiency have been made full use so as to have the moderate characteristics in the conventional arrangement as described above because the nozzle is arranged to have a straight structure. Thus, there is a limit to further improvement both of the ejection efficiency and the refill efficiency. It should be noted that U.S. Pat. No. 6,158,843 described above discloses an arrangement in which a space and a fluid resistant protruding element are disposed in a supply chamber or in the vicinity of a supply port in order to increase a refill speed and to locally reduce and increase an ink flow path. However, this patent focuses attention only on the improvement of the flow of ink, which is supplied from a supply chamber, in each nozzle and does take into consideration the improvement of the ejection efficiency of, in particular, a nozzle.